The mode of failure of cement boards is commonly attributed to the alkali attack of the fiberglass scrim used to reinforce the board. These scrims are coated with plasticized PVC (i.e., plastisols) and, while it is commonly accepted that plastisols provide adequate protection of the fiberglass, these coatings provide only limited protection for a period of time. Plastisols are the most commonly used coating for this market as the price and performance meet the minimum requirements established by the industry.
The cement backer board market typically utilizes PVC-coated glass scrims to provide strength and stiffness for handling and use purposes. The strength of these boards is directly affected by the durability of the scrim. Scrims produced by either a non-woven process (Chomarat North America, Chomarat, Adfors, Dewtex, Milliken, Kirson) or a woven process (Phifer Wire) typically have, within certain limits, nearly the same performance in the alkaline cement matrix. It is assumed that each manufacturer, while having their own proprietary formula, has a basic plastisol which results in similar performance in the cement board. With similar durability performance, the construction (yarns per inch in each direction) from each scrim manufacturer has to be nearly identical to provide the necessary strength, initial and aged, as specified by the cement board manufacturer and set forth by the industry standard as specified in ASTM C-947 (Standard Test Method for Flexural Properties of Thin-Section Glass-Fiber-Reinforced Concrete).
In addition, the scrim industry has typically looked toward the fiberglass yarn manufacturer to provide suitable binder chemistry (e.g., a sizing composition) on the yarn that would result in compatibility with the PVC coating and, hopefully, some degree of additional alkali resistance. The yarn manufacturer is limited to how much binder (e.g., a sizing composition) they can put on the fiberglass (typically less than 2%) which can include not only a silane for compatibility with the coating or polymer system, but a starch and lubricant package as well. Higher levels of such “add-ons” are believed to generally reduce the ability of the yarn to be subsequently processed.
Accordingly, there continues to be an industrial need for plastisol compositions exhibiting improved performance which would, preferably, also allow changes to be made in the construction of a reinforcing scrim (particularly for use in cement boards) to lower the cost and, therefore, the selling price.